Photoacoustic measuring apparatus

ABSTRACT

A photoacoustic measuring apparatus includes a light source, a movable holding unit which holds an object, a light diffusing unit which fixes the distance between the light diffusing unit and the holding unit and diffuses light incident from the light source, and an acoustic wave obtaining unit which obtains an acoustic wave generated from the object by the light emitted via the holding unit and the light diffusing unit.

RELATED APPLICATIONS

This application is a divisional of application Ser. No. 14/193,117,filed Feb. 28, 2014, which is a divisional of application Ser. No.13/151,543, filed Jun. 2, 2011, claims benefit of the filing dates ofthose applications under 35 U.S.C. § 120, and claims priority benefitunder 35 U.S.C. § 119 of Japanese patent application 2010-132860, filedJun. 10, 2010. The entire contents of each of the three mentionedapplications are incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

In recent years, photoacoustic tomography, which uses the characteristicof ultrasound waves being less diffusible in a living body than light tocalculate an optical characteristic value distribution in the livingbody at high resolution, has been proposed. In this specification,hereinafter, photoacoustic tomography will be referred to as “PAT”. In ameasuring apparatus using the principle of PAT, when pulsed lightgenerated from a light source irradiates a living body, it is propagatedwhile diffusing in the living body. An absorber included in biologicaltissue absorbs the energy of the propagated pulsed light to generate anacoustic wave (the acoustic wave is also called a “photoacoustic wave”,and is typically an ultrasound wave). An acoustic wave signal obtainedby detecting and signal-processing the acoustic wave is subjected to ananalyzing process, so that an optical characteristic distribution, inparticular, an optical energy absorption density distribution, in theliving body can be obtained.

In PAT, the sound pressure (P) of the acoustic wave obtained from theabsorber in the living body by light absorption can be expressed by thefollowing equation (1):P=Γ·μa·Φ  Equation (1)

Here, Γ is a Grüneisen coefficient which is an elastic characteristicvalue, and is obtained by dividing the product of the coefficient ofcubic expansion (β) and the square of the sound speed (c) by thespecific heat (Cp). μa is the absorption coefficient of the absorber,and Φ is the light amount in a local region (the light amountirradiating the absorber).

In recent years, breast diagnosis has been studied as a possibleapplication of PAT to living bodies. In this specification, use of PATapparatus for breast diagnosis will be referred to as “photoacousticmammography” (or “PAM”). A PAM apparatus is an apparatus which imagesangiogenesis formed around a tumor at the time the tumor is formed, anda region having high absorption coefficient and including theangiogenesis, thereby detecting the tumor position in a breast. Todiagnose the entire breast by PAM, measurement is required to bepossible in a deep region, even at a depth above 4 to 5 cm below thesurface of the patient's body. Although the acoustic wave signalintensity is in proportion to the light amount Φ the beam incident onthe biological tissue is diffused, with the result that the light amountreaching the deep portion decreases exponentially with depth. For thisreason, to enable depth observation, light preferably irradiates a widerange at as high an irradiation intensity as is permissible for livingbodies (the “MPE”, or maximum permissible exposure). Therefore, as thelight source, a high-output flash lamp excitation solid-state laser istypically used.

The flash lamp excitation solid-state laser has a locally high energyoutput distribution, the light amount distribution in a beam being lessuniform than a semiconductor laser or a He—Ne laser. When a beam atlocally high energy density is used, only part of it is at the upperlimit of irradiation intensity. Therefore, the light amount distributionof a beam is preferably made uniform using a light modulation membersuch as a diffusing plate (see U.S. Patent Application Publication No.2006/0184042).

SUMMARY OF THE INVENTION

In PAM having a breast holding mechanism which fixes an observed portionaccording to the shape of a breast, unlike X-ray mammography, thefollowing problems arise. In X-ray mammography, since X-rays travelsubstantially straight in a living body, the X-ray intensitydistribution in the living body cannot be affected by the position ofthe parallel plates of the breast holding mechanism.

On the other hand, in PAM, when a light diffusing plate is installed ina beam propagation path in order to make the light amount distributionin the beam uniform, the outgoing beam diverges increasingly, inaccordance with the propagation distance. For this reason, when thedistance from the surface of the patient's body to the diffusing plateis different at the time of measurement of an object at plural points,the size of the irradiation region becomes different. As a result, theirradiation intensity at each point is changed according to the breastfixing position, which deteriorates the photoacoustic signal.

The present invention has been made in view of the above problems, andan object of the present invention is to provide a photoacousticmeasuring apparatus which can irradiate the surface of a living bodyuniformly and efficiently.

This invention provides a photoacoustic measuring apparatus comprising:

a light source;

a movable holding unit which holds an object;

a light diffusing unit which fixes the distance between the lightdiffusing unit and the holding unit and diffuses light incident from thelight source; and

an acoustic wave obtaining unit which obtains an acoustic wave generatedfrom the object by the light emitted via the holding unit and the lightdiffusing unit.

According to the photoacoustic measuring apparatus of the presentinvention, the surface of a living body can be irradiated uniformly andefficiently.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a front detection type PAT apparatus;

FIG. 2 is a schematic diagram of a rear detection type PAT apparatus;

FIG. 3 is a schematic diagram of a both-side (bilateral) irradiationtype PAT apparatus;

FIGS. 4A and 4B are diagrams each showing an irradiation region when aliving body holding mechanism including a diffusing mechanism isprovided;

FIGS. 5A and 5B are further diagrams each showing an irradiation regionwhen a living body holding mechanism including a diffusing mechanism isprovided;

FIGS. 6A and 6 b are diagrams each showing the irradiation region of acomparative example in which a diffusing mechanism is fixed;

FIG. 7 is a schematic diagram of a front detection type PAT apparatusaccording to a first example;

FIG. 8 is a schematic diagram of a rear detection type PAT apparatusaccording to a second example; and

FIG. 9 is a schematic diagram of a bilateral irradiation type PATapparatus performing volume diffusion according to a third example.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to the drawings. However, the size, material,shape, and relative arrangements of components described below should beappropriately changed according to the configuration of the particularapparatus to which the present invention is applied and to the actualconditions obtaining, and the scope of the present invention is notintended to be limited to the following description.

A photoacoustic measuring apparatus has a living body holding mechanismwhich fixes the measured portion of an object. As breast fixing methodsin breast diagnosis, a method for fixing the side surfaces of a breastusing two parallel plates, a method for entirely pressing and fixing thefront surface of a breast, and a method for arcuately fixing a portionaround a breast have been proposed. In particular, a breast holdingmechanism using a parallel plate has the advantage of being arbitrarilymovable according to the size of the patient's breast. In addition, thebreast holding mechanism using a parallel plate is adapted to X-raymammography, and thus has another advantage in that image comparisonwith X-ray mammography is easy.

When a breast is fixed using a parallel plate in PAM, some lightirradiation directions and ultrasound detector arranging directions withrespect to the parallel plate can be considered. This will be describedwith reference to FIGS. 1 to 3. Further, in the description of these, itshould be borne in mind that two parallel plates, not shown, whichsandwich and hold a breast are present.

FIG. 1 shows a front detection type apparatus in which an ultrasounddetector 104 is arranged on the opposite side of irradiation light 103with respect to an object 101. The irradiation light is emitted bydiffusion in a living body, as shown in a diffused light area 102. FIG.2 shows a rear detection type apparatus in which an ultrasound detector204 is arranged on the same side as irradiation light 203 with respectto an object 201. The irradiation light is emitted, as shown in adiffused light area 202. FIG. 3 shows a bilateral irradiation typeapparatus in which irradiation light 303 b is emitted from the oppositeside of an ultrasound detector 304 with respect to an object 301, andirradiation light 303 a is emitted from the same side as the ultrasounddetector 304 with respect to the object 301. The irradiation light isemitted, as shown in a diffused light area 302.

In the parallel plates used to sandwich and hold the breast, variousmovable forms of moving each of the parallel plates on the ultrasounddetector side and on the opposite side independently, moving only(either) one of them, and the like can be selected. The movable parallelplates can appropriately press and hold the breast even given thevariations that exist among individuals.

In the irradiation used in PAM, the locally high irradiation energydensity region can be removed by using a light diffusing mechanism.Further, the irradiation intensity of the entire irradiation region canbe made uniform with uniformity according to diffusion angle. Theirradiation range of a beam which has passed through the light diffusingmechanism is diverged according to diffusion angle. Here, with the useof the movable parallel plates for fixing the breast, when the lightdiffusing mechanism is fixed, the distance from the light diffusingmechanism to the parallel plate, that is, the beam propagation distance,is changed with the movement of the plate, so that the irradiationregion is changed.

By the way, the permissible light amount in the irradiation onto thesurface of a living body is defined according to irradiation energy orirradiation amount per unit area. For this reason, when the irradiationregion is varied to increase the irradiation area, the light amount perunit area onto the surface of a living body is decreased. On the otherhand, when the irradiation area is decreased, the light amount per unitarea onto the surface of a living body is increased, so that theirradiation intensity can be equal to or larger than MPE (maximumpermissible exposure). In PAM which requires depth observation, it ispreferred that the irradiation intensity be constant within the safeirradiation light amount range equal to or smaller than MPE and be highso that depth observation is enabled. For this reason, the irradiationregion is required to be constant regardless of the movement of themovable parallel plate.

A method for making the irradiation region constant in the presentinvention will be described below. In FIGS. 4A and 4B, light 402 isemitted from a light source (not shown). In the breast holding mechanismshown in FIGS. 4A and 4B, a light diffusing mechanism 404 is provided toa parallel plate 403 itself. FIGS. 4A and 4B show, in the breast holdingmechanism, two types of configurations, in which the interval betweenthe parallel plates is different. As shown in these drawings, theirradiation region formed by the beam via the light diffusing mechanismis not changed, and has a constant area regardless of the intervalbetween the parallel plates 403. Alternatively, even when the movableparallel plate is moved to press and hold the breast, the irradiationregion cannot be changed as between before and after movement.

To realize the light diffusing mechanism, there is a method for makingthe light amount distribution of a beam uniform by providing a surfacediffusing mechanism in which the surface shape on the incident side isroughened like ground glass. However, when ground glass is used, much0-order light is included so that a beam perpendicularly incident uponthe substrate is perpendicularly outgoing therefrom. For this reason, toincrease the uniformity of the light amount distribution, the use of aholographic diffusing plate in which a micron-level surface structure insurface relief hologram pattern is randomly arranged on the surface ofthe substrate is effective. The uneven shape of the surface can bepattern-formed by epoxy UV curing. The diffusion angle can be selectedaccording to non-periodic uneven shape, so that the beam can be diffusedat high transmissivity without depending on the visible to infraredwavelengths.

In addition, the light diffusing mechanism can also use volumediffusion, in which particles having different refractivities areincluded in the parallel plate substrate for diffusion control in theparallel plate. In particular, volume diffusion can be done by includingtitania particles in an acrylic or polycarbonate substrate. Thediffusion angle can be increased, not only by including the particlesuniformly in the substrate, but also by increasing the amount ofparticles included from the incident side to the outgoing side toincrease the diffusion coefficient. When laser light having highcoherence is used, the beam can be locally focused near the surface inthe substrate due to refraction on the surface of the substrate. Thediffusion is increased gradually, to lower the possibility that the beammight be focused, so that the light can be diffused uniformly.

In this way, the light diffusing mechanism of the parallel plate isreferred to as the light diffusing mechanism provided on the surface ofthe plate or inside the plate. As a result, the entire plate has adiffusion angle defined in a predetermined range. When the diffusionangle is shown as full width at half maximum, the entire light diffusingmechanism (light diffusing unit) of the present invention has adiffusion angle of 0.5□ or more. The diffusion angle is preferably 1□ ormore, and more preferably is 5□ or more. Such light diffusing mechanismemits uniform light onto the object.

In addition, as shown in FIGS. 5A and 5B, a jig 507, which is a fixingmember which holds the interval between the light diffusing mechanism504 and parallel plate 503 constant, can be provided. In this case,since the jig having the diffusing mechanism can be moved togetheraccording to the movement of the holding mechanism, the irradiationregion 505 can be constant. For instance, even when the state of FIG. 5Ais changed to the state of FIG. 5B in which the parallel plate is movedin the direction pressing and holding the breast, the irradiation areaon the surface of the breast is not changed. In the configuration ofFIGS. 5A and 5B, unlike the case in which the light diffusing mechanismis provided on the parallel plate, the surface diffusing mechanism isprovided on the light-outgoing side.

However, the light diffusing mechanism 504 may be a member using avolume diffusing mechanism in addition to the member using the surfacediffusing mechanism. In addition, when the diffusing mechanism isprovided on the surface of the light diffusing mechanism 504 installedin the space in which the light is propagated, the diffusing surface maybe the incident side or the outgoing side. However, when laser lightthat is large in light amount and high in coherence is used, the beamcan be locally focused near the surface in the parallel plate byrefraction on the surface of the parallel plate. Therefore, in the lightdiffusing member using a material with low damage resistance, theoutgoing side is preferably the diffusing surface.

FIGS. 6A and 6B show the configurations of a comparative example withrespect to the forms of FIGS. 5A and 5B. In these configurations, theposition of a light diffusing mechanism 604 is fixed, and the distancebetween the light diffusing mechanism and the plate is increased as theparallel plate is moved in the direction pressing the object. In thiscase, with the movement of the parallel plate, an irradiation region 605of the diffused beam is changed (enlarged) from FIG. 6A to FIG. 6B. As aresult, the irradiation intensity in the irradiation region issignificantly lower than MPE, thereby lowering the measuring efficiency.

First Example

In this example, a configuration example of a front detection type PATapparatus to which the present invention is applied will be described.

As shown in FIG. 7, the front detection type PAT apparatus having planarparallel plates 705 a and 705 b, an ultrasound detector 704, and a lightsource 707 which emits irradiation light 703 is prepared for abiological tissue 701 as an object. The emitted light is diffused toirradiate a diffused light area 702. In addition, in this apparatus, theultrasound detector scans the surface of the plate 705 a. The beam onthe opposite side is moved with the movement of the ultrasound detectorto irradiate the front surface (the left side on the drawing sheet) ofthe ultrasound detector 704 at all times.

As the light source, a Nd:YAG laser, which is a pulse laser having anoscillation wavelength of 1064 nm, is used. Other than this, thewavelength band from the visible to the infrared region of about 500 nmto 1400 nm can be used. The wavelength varying technique using TI:sa(titanium-sapphire) and OPO (optical parametric generation) usedtogether with the Nd:YAG laser and an alexandrite laser using analexandrite crystal which oscillates in the wavelength band of about 750nm can also be used. As the parallel plates 705 a and 705 b,polymethylpentene having a refractivity of 1.46 is used, the thicknessbeing 10 mm.

In this example, a holographic diffusing mechanism which is the surfacediffusing mechanism is provided on the incident side of the plate 705 bon the opposite side of the plate 705 a on which the ultrasound detector704 is arranged, that is, on the outside of the plate that does notcontact the patient's breast. For that, a sheet 706 having theholographic diffusing mechanism adheres to the parallel plate 705 b.Alternatively, the surface of the plate may be directly processed toform the holographic diffusing mechanism. That is, the surface diffusingmechanism may be part of the plate, or may be a member different fromthe plate. The parallel plate corresponds to the “holding unit” in theembodiments described above, and similarly, the sheet having theholographic diffusing mechanism corresponds to the “light diffusingunit”.

The diffused light is absorbed into the biological tissue 701 togenerate an ultrasound wave (acoustic wave) by expansion and contractionof the biological tissue. This is obtained by the ultrasound detector704. The ultrasound detector includes, e.g., a piezoelectric device, andcan convert the obtained ultrasound wave to an electric signal which isused for image reconstruction, that is, to produce an image containinginformation about the biological tissue. The ultrasound detectorcorresponds to the “acoustic wave obtaining unit” referred to above.

With the use of the PAT apparatus having such configuration, one or bothof the parallel plates 705 a and 705 b are moved according to the sizeof the breast, to hold the breast for measurement, so that the distancefrom the light diffusing mechanism to the object is constant. As aresult, the variation of the irradiation region by light diffusion canbe inhibited, so that irradiation can be performed effectively.

Second Example

In this example, a configuration example of a rear detection type PATapparatus to which the present invention is applied will be described.

As shown in FIG. 8, the rear detection type PAT apparatus havingparallel plates 805, an ultrasound detector 804, and irradiation light803 of a light source (not shown) is prepared for a biological tissuesample (e.g., a patient's breast) 801. The same light source as thefirst example can be used. The light emitted from the light source isdiffused to irradiate a diffused light area 802.

A film larger than the detector surface and subjected to acousticmatching is fixed on the front surface of the ultrasound detector. Thefilm preferably has higher ultrasound wave transmissivity, and requireslight transmissivity. In this example, a polycarbonate film having athickness of 200 □m is used. A unit which integrates the ultrasounddetector and the polycarbonate film is contacted with the plate holdingthe breast via a solution, such as castor oil, which becomes an acousticmatching layer which can easily pass an ultrasound wave therethrough,and is moved in parallel. As the light diffusing mechanism, a surfacediffusing mechanism 806 which is the holographic diffusing mechanism isprovided on the polycarbonate film. The surface diffusing mechanism isprovided on a region other than the contacted portion of thepolycarbonate film and the ultrasound detector 804, so that the beam isincident from the region.

In measurement performed using the PAT apparatus having suchconfiguration, even when the parallel plate 805 is moved according tothe size of the breast to hold the breast, the distance from the lightdiffusing mechanism to the object is held constant. As a result,variation of the irradiation region by light diffusion can be inhibited,so that irradiation can be performed effectively.

Although the rear detection type PAT apparatus has been described here,the method of this example is combined with the method of the firstexample to emit the light from the light source onto the opposite sideof the ultrasound detector, so that a both-side irradiation type PATapparatus can be realized.

In addition, the configuration of this example can also include oneplate by eliminating the plate on the opposite side of the ultrasounddetector. In that case, the breast is pressingly fixed and held formeasurement. This configuration can also inhibit the variation of theirradiation region by diffusion, so that irradiation can be performedeffectively. In addition, when the breast is pressingly fixed by an arcmember, the present invention is applicable.

Third Example

In this example, a configuration example in which the light diffusingmechanism is realized by volume diffusion in place of surface diffusionused in the above examples will be shown.

FIG. 9 shows the configuration of the PAT apparatus of this example.Here, the polycarbonate film which provides surface diffusion to thefront surface of the ultrasound detector, as shown in the above example,is not necessary. A biological tissue 901 such as a breast is theobject, and is sandwiched and held between parallel plates 905.Irradiation lights 903 a and 903 b led by an optical system from a lightsource (not shown) irradiate the object and are diffused onto a diffusedlight area 902. An ultrasound detector 904 detects an acoustic wavegenerated from the patient.

In this example, to adjust the refractivity in the polymethylpenteneresin which is the material of the parallel plates 905, titaniaparticles are included. Two titania inclusion methods in which titaniaparticles are uniformly included in the plates, and in which the rate oftitania particles included is graduated, so as to gradually increase thediffusion effect from the incident side to the outgoing side of thebeam, are prepared.

Whichever of these two types of plates is used, the parallel plate 905is moved according to the size of the breast using the PAT apparatushaving such configuration to hold the breast for measurement, thedistance from the light diffusing mechanism to the object can be heldconstant. As a result, variation of the irradiation region by lightdiffusion can be inhibited, so that irradiation can be performedeffectively. Between these two types of plates, the one in which theamount of titania is gradually increased from the incident side, may bepreferable in that the beam is prevented from being locally focused nearthe surface by refraction.

As another form of this example, a configuration example in which thediffusing mechanism moving with the breast holding mechanism is providedmay be adopted. In this case, the interval between the parallel platesas the breast holding mechanism is changed depending on the size of thebreast. The movable parallel plate is coupled to the member having thediffusing mechanism to hold the distance between the plate and thediffusing member. As in the first example, since the beam on theopposite side is in-plane operated with the ultrasound detector at thetime of acoustic wave signal measurement, the diffusing member hassufficiently large vertical and horizontal sizes, so that the beam canbe emitted via the diffusing member at all times.

By such configuration, the variation of the irradiation region bydiffusion can be inhibited, so that irradiation can be performedeffectively.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

What is claimed is:
 1. A photoacoustic measuring apparatus comprising: aholding unit configured to hold an object; a light emitting unitconfigured to emit light generated by a light source; a diffusing plateconfigured to diffuse the light emitted from the light emitting unit toirradiate the object with the diffused light; and an acoustic waveobtaining unit configured to obtain an acoustic wave generated from theobject by the irradiation, wherein the holding unit and the diffusingplate are integrated, wherein the photoacoustic measuring apparatus isconfigured such that the light entering the diffusing plate from a firstprincipal surface of the diffusing plate exits from a second principalsurface of the diffusing plate and irradiates the object, wherein thesecond principal surface is opposed to the first principal surface. 2.The photoacoustic measuring apparatus according to claim 1, wherein anarea of an irradiation region on a surface of the object contacting theholding unit is kept constant even when the holding unit moves.
 3. Thephotoacoustic measuring apparatus according to claim 1, wherein thediffusing plate is provided with a surface diffusing mechanism on alight outgoing side.
 4. The photoacoustic measuring apparatus accordingto claim 1, wherein the diffusing plate comprises a volume diffusingmechanism.
 5. A photoacoustic measuring apparatus comprising: a holdingunit configured to hold an object; a light emitting unit configured toemit light generated by a light source; a diffusing plate configured todiffuse the light emitted from the light emitting unit to irradiate theobject with the diffused light; and an acoustic wave obtaining unitconfigured to obtain an acoustic wave generated from the object by theirradiation, wherein the holding unit and the diffusing plate areintegrated, wherein the photoacoustic measuring apparatus is configuredsuch that the light entering the diffusing plate from a first principalsurface of the diffusing plate exits from a second principal surface ofthe diffusing plate and irradiates the object, and wherein a diffusioncoefficient inside the diffusing plate increases with distance from thefirst principal surface.
 6. The photoacoustic measuring apparatusaccording to claim 5, wherein an area of an irradiation region on asurface of the object contacting the holding unit is kept constant evenwhen the holding unit moves.
 7. The photoacoustic measuring apparatusaccording to claim 5, wherein the diffusing plate is provided with asurface diffusing mechanism on a light outgoing side.
 8. Thephotoacoustic measuring apparatus according to claim 5, wherein thediffusing plate comprises a volume diffusing mechanism.